It is known that sucrose and betaine are recoverable from molasses by chromatographic separation. U.S. Pat. No. 4,359,430 to Suomen Sokeri Oy, describes a chromatographic method for the recovery of betaine from molasses by a batch process in which diluted molasses is fractionated with a polystyrene sulphonate cation exchange resin in alkali metal form. This method achieves good separation of sucrose and betaine. This reference also discloses a method in which a betaine-enriched fraction obtained from a first fractionation is subjected to further chromatographic purification. The further purification step is said to be capable of separating other components from the betaine-enriched fraction. However, the dry solids content in the sucrose and betaine fractions obtained by this method is relatively low, therefore, large amounts of eluant water must be evaporated when recovering the sucrose and betaine from the respective fractions by crystallization.
Continuously operated chromatographic separation processes presently employ the SMB method, which method is used in a variety of different applications. The SMB method has a separating performance that is several times higher than that of the batch method, and also results in significantly lower dilution of the products or, conversely, lower consumption of eluant.
The SMB method may be carried out in either a continuous or a sequential mode. In both processes, the pattern of fluid streams is the same. These streams are (1) the supply of feed solution and eluant to the bed or beds, (2) the recycling of the liquid mixture between beds, and (3) the withdrawal of products from the beds. The flow rate for these flows may be adjusted in accordance with the separation goals, i.e., increased yield, purity, or capacity. In a continuous SMB process, which was first disclosed in the early 1960s in U.S. Pat. No. 2,985,589, all fluid streams flow continuously. Separation of sucrose by such continuous SMB methods has been described in international publication no. WO 91/08815 by the Amalgamated Sugar Company and in U.S. Patent 4,990,259 to M. Kearney and M. Mumm and assigned to The Amalgamated Sugar Company.
In a sequential SMB process, some of the fluid streams do not flow continuously. Sequential SMB fractionation methods in which a sucrose fraction and a betaine fraction are recovered from beet molasses are disclosed in Finnish Patent 86,416 to Suomen Sokeri Oy, which corresponds to U.S. Pat. No. 5,127,957 and international publication no. WO 94/17213 to Suomen Sokeri Oy. German Offenlegungsschrift 4,041,414 to Japan Organo Co., which corresponds to British published application 2,240,053, also discloses a sequential SMB method by which several product fractions are recovered from sugarbeet molasses.
In the sugar industry, the important parameters in the fractionation of molasses to recover sucrose include the purity and yield of sugar, the separation capacity, and the eluant/feed ratio. A purity of 92% and a yield of 90% are the usual requirements for a sugar product. In order to increase the capacity to recover sugar from the process, the flow rates, which are generally higher in SMB processes than in batch processes, are increased. Along with the increase in the flow rate, however, a "flat tail" is produced in the sucrose elution profile. A "flat tail" means that the concentration of sugar has not peaked sharply, but, rather, has been considerably diluted with the increased volume of eluent. This is especially disadvantageous to the recovery of a second dissolved component. With respect of the recovery of sucrose and betaine, this effect is shown in the elution profiles presented in U.S. Pat. No. 4,359,430 and Finnish Patent 86,416, for example. In the course of obtaining a high sucrose yield, the betaine yield is diminished because part of the betaine is allowed to pass into the sucrose fraction wherefrom it is removed in the sucrose crystallization step. Likewise, if a high betaine yield is desired, considerable amounts of sucrose end up in the betaine fraction, thus diminishing the sucrose yield and considerably impairing the purity of the betaine fraction.
In the above references, the purity of the betaine fraction obtained by the process of German Offenlegungsschrift 4,041,414 is relatively good, 80.9% on a dry solids basis (d.s.), but the purity of the sucrose fraction, 87% d.s., is inadequate for the sugar industry. It can be concluded from the composition of the feed solution of Example 3 in the reference that the "thin juice" was demineralized prior to the SMB fractionation by the "KAAK method" (which refers to cation exchange-anion exchange-anion exchange-cation exchange as described in Sayama, K., Kamada, T., and Oikawa, S., Production of Raffinose: A New By-Product of the Beet Sugar Industry, British Sugar plc, Technical Conference, Eastbourne 1992). Molasses produced by such a beet sugar process has a different composition from common molasses. Typically, beet molasses contains 1.5-3.50 by weight of raffinose and 3.5-6.5 weight % of betaine on a dry solids basis. Since the feed solution of Example 3 in German Offenlegungsschrift 4,041,414 has a raffinose content of 17.3% by weight and a betaine content of 12.26 by weight on a dry solids basis, it can be concluded, on the basis of the raffinose-to-betaine ratio, that roughly half of the betaine contained in common beet molasses was lost in the ion exchange treatment.
According to Finnish Patent 86,416, a purity as high as 70.9% d.s. for the betaine fraction was obtained, with 11.1% d.s. of sucrose present in the betaine-enriched fraction. However, the 86.8 a purity of the sucrose fraction does not meet the requirements of the sugar industry. Similarly, the 47.5% purity of the betaine fraction reported in international publication no. WO 94/17213 is rather poor.